Mechanical wave filter



H. A. BURGESS MECHANICAL WAVE FILTER April 17, 1928.

Filed April 22, 1924 Patented Apr. 17, 1928.

UNITED STATES HARRY A. BURGESS, OF NEW YORK, N. Y., ASSIGNOB TO WESTERN ELECTRIC COM- PATENT OFFICE.

rm, INCORPORATED, OF NEW YORK, N. Y,, A CORPORATION OF NEW YORK.

MECHANICAL WAVE FILTER.

Application filed April 22, 1924. Serial No. 708,127.

electrica systems are:

Mechanical. Electrical. Force Electromotive force Displacement Charge Velocity Current Mass Inductance Elasticity Reciprocal of capacity Friction Resistance Electrical filters generally comprise a reiterative series of sections which may be uniform through the entire filter network but which in practice are generally made up of different types of electrical networks. The theory and design formulae for them are disclosed in U. S. Patent No. 1,227,113 granted to G. A. Cam bell, May 22, 1917, and in an article by tto J. Zobel in. the Bell System Technical Journal for January, 1923 (Vol. II No. 1) pp. 1 to 46. The theory and design formulae contained in these publications are equally applicable to mechanical filters, bearing in mind that each electric quantity in these formulae should be replaced by the corresponding uantity of the mechanical system as given a ove.

The ideal electrical filter would have lumped inductances and capacities free from resistance. Similarly, the ideal mechanical filter would have rigid masses and massless springs connected by massless connections and moving without friction. In practice, of course, these conditions can never be realized but in the design of any filter they can be approximated sufiiciently closely to give the results desired.

In mechanical filters, the energy is transmitted by space vibrations of the elements. The vibrations may be, for example, rectilinear either transverse or longitudinal of the filter or curvilinear (such as torsional) or a combination of rectilinear and curvilinear motion.

In general, mechanical wave filters consist of a series of rotatable or vibratable members coupled together by other members possessing mass or elasticity or both. The first mentioned members correspond to the series elements of the analogous electrical network while the coupling members correspond to the shunt elements. The action of mechanical wave filters is somewhat more readil 1 understood when keeping in mind the analogy to the electrical network and it is therefore often desirable to use the terms series and shunt when referring to the elements of a mechanical filter.

It is an object of this invention to im rove the construction of mechanical wave lters.

It is a further object of this invention to improve the coupling elements used in mechanical wave filters.

Itis somewhat diflicult to provide coupling elements which do not possess mass and elasticity in undesirable locations; that is, so as to glve complicated networks which are difficult to design and whose characteristics are diflicult to forecast. One feature of this invention comprises the use of members which have motion in two different directions or two difierent types of motion so that the effective value of the member in one motion can be used as the value of the series element and the effective value in the other motion can be used as the shunt element. For example, according to one form of this invention, rotatable coupling members having appreciable mass are mounted on vibrating or rotating series members which may or may not have appreciable mass and the coupling members are attached to the adjoining series members so as to transmit motion irom one to the other. The effective coupling value of such a member is due to its moment of inertia while its efiect as a series element is due to its mass. Thus, the member may comprise a small wheel having its mass concentrated in its rim so as to give a high moment of inertia for a low mass and consequently a high ratio of shunt to series mass in the filter; or by varying the distribution of the mass the ratio may be changed.

By using rotatable coupling elements, it is also possible to build a mechanical wave filter which is both simple and compact in construction.

This invention can be more readily understood by reference to the following description in connection with the drawing in which Figs. 1,2 and 3 show three mechanical filters embod ing the invention and Figs. 1, 2, and 3 5 10w diagrammatically the analogous electrical filters.

The filter shown in Fig. 1 comprises a series of reeds or steel bars 5 rigidly clamped at their lower ends between the bed plates 6 and 7. The reeds are coupled together by rotatable elements or wheels 8 mounted on pivots 9 attached to the reeds at their free ends. The wheels 8 are provided with notches 10 into which fit the pins 11 which are fixed to the adjoining reeds. This filter is the mechanical analogue of the electrical wave filter, one section ofwhich is shown in Fig. 1*. The mass and reciprocal of the elasticity of the reeds 5 correspond, respectively, to the series inductance, L and the series capacity, 0 of the electrical filter and similarly, the coupling mass 8 corresponds to the shunt inductance L As explained above, the series mass may be largely the mass of the wheel 8 when its value is so high that the distributed mass of the reed may be neglected or it may be a combination of both the mass of the wheel and the distributed mass of the reed, while the shunt mass is really the moment of inertia of the wheel.

In the operation of the filter, one of the reeds is set in motion by vibrations imparted to the arm 12 and the vibrations of the frequencies in the transmitted band are transferred from one reed to the next throu h the coupling masses 8 and finally passer? with practically uniform attenuation to the output circuit, the arm 13 serving to transfer the vibrations from the last reed to the output mechanism. Vibrations of all other frequencies are suppressed or highly attenuated by passage through the network. One Way of briefly explaining the action of the filter is this: The upper edge of the transmitted band is the resonant fre quency of one of the series masses (which, as stated, includes the mass of the reed and the mass of the coupling disk) vibrating independently under control of its own restoring spring. The attenuation of all the higher frequencies is due to the resonant effect of the series arms. The attenuation of the lower frequencies may be considered as due to the shunting effect of the shunt arms, i. e., to the inability of the coupling masses to transmit low frequency vibrations between the series elements, the masses rotating when low frequencies are impressed on the filters, thus causing a slip between sections. The ratio of the frequency of the upper edge of a band to the frequency of the lower edge of the band is determined by the ratio of the effective coupling mass to the series mass and decreases as the ratio of these masses decreases.

The effective coupling mass is equal to the moment of inertia of the coupling element about its axis of rotation divided by the square of the distance from that axis of the location of the coupling pin. 7

Fig. 2 shows a type of filter which embodies the mechanical analogue of an antiresonant circuit in the shunt arm as is indicated in the electrical filter shown in Fig. 2. ing the pins 11 on the ends of springs 14 which are attached to the adjoining reeds instead of mounting the pins on the reeds themselves as was done in the filter shown in Fig. 1. The springs 14 are made very light so that for design purposes they can be considered massless,

The analogy between the elements of mechanical and electrical filters is similar to that of Figs. 1 and 1 the mass and re ciprocal of elasticit of the series members corresponding to t e series inductance L and capacity C of the electrical filter and the moment of inertiaof the wheel 8 and the reciprocal of the elasticity of spring 14 corresponding to the shunt inductance L and shunt capacity C of the electrical filter.

Fig. 3 shows another type of band pass filter. This particular type comprises a series resonant circuit in each of the series and shunt arms of the filter. The construction of this filter is similar to that of Fig. 1 except that a spiral spring 15 is attached to the wheel 8 and the pins 9 much in the same way as the hairspring of a watch. The reciprocal of the elasticity of this spring corresponds to the shunt capacity C of the electrical filter.

It is obvious that this invention may be applied to various other types of filters and is not limited to the specified type shown, for example, the rotating coupling elements could be used between rotatable sections instead of the vibrating sections shown, and various modifications can be made in the construction without departing from the spirit of the invention as defined in the appended claims.

N o intention is made to limit the invention to the particular types of filters shown for only the simple networks have been illustrated. The method of combining these simple structures to form composite filters or multi-element filters is obvious to anyone skilled in the art.

What is claimed is:

1. A mechanical wave filter comprising a plurality of series members and a plurality of rotatable coupling members for transferring vibrational ener from one series member to the next, sa1d coupling members having an appreciable moment of inertia so as to offer an impedance to the vibrations which varies in amount with the frequency of the vibrations.

2. A mechanical wave filter comprising a member having freedom of movement in. a

This type of filter is obtained by mountplurality of manners so that a characteristic of said member as regards motion in one manner acts as one element of the filter and a characteristic of the member as regards its movement in another manner acts as a second element of the filter.

3. A mechanical wave filter comprising a member having freedom of movement in a plurality of planes so that a characteristic of said member as regards its movement in one plane acts as one element of the filter and a characteristic of said member as regards its movement in another plane acts as a second element of the-filter.

4. A mechanical wave filter comprising a succession of members each having freedom of rotation and freedom of translatory movement, the moment of inertia of a said member acting as one element of the filter and the mass of said member acting as another element of said filter.

5. A mechanical wave filter comprising a member having freedom of rotation in one plane and freedom of translatory movement in a second plane so that the moment of inertia of said member as regards rotation in said first plane acts as one element of the filter and a characteristic of said member as regards its movement in said second plane acts as a second element of said filter.

6. A mechanical wave filter having series impedance to motion and shunt impedance to motion comprising a member having freedom of rotation and freedom of translatory movement so that the value of one of said impedances depends upon the moment of inertia of said member and the value of the other of said impedances depends upon the mass of said member.

7. A mechanical wave filter comprising a plurality of oscillatable members and a plurality of rotatable members for coupling together said first mentioned members, the series impedance of said filter depending upon the mass of said rotatable members and the shunt impedance of said filter depending upon the moment of inertia of said rotatable members.

In witness whereof, I hereunto subscribe my name this 16th day of April, A. 1)., 1924. so

HARRY A. BURGESS: 

